254 research outputs found

    Precision Weak Gravitational Lensing Using Velocity Fields: Fisher Matrix Analysis

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    Weak gravitational lensing measurements based on photometry are limited by shape noise, the variance in the unknown unlensed orientations of the source galaxies. If the source is a disk galaxy with a well-ordered velocity field, however, velocity field data can support simultaneous inference of the shear, inclination, and position angle, virtually eliminating shape noise. We use the Fisher Information Matrix formalism to forecast the precision of this method in the idealized case of a perfectly ordered velocity field defined on an infinitesimally thin disk. For nearly face-on targets one shear component, γ×\gamma_\times, can be constrained to 0.00390I025npix0.003\frac{90}{I_0}\frac{25}{n_{\rm pix}} where I0I_0 is the S/N of the central intensity pixel and npixn_{\rm pix} is the number of pixels across a diameter enclosing 80\% of the light. This precision degrades with inclination angle, by a factor of three by i=50∘i{=}50^\circ. Uncertainty on the other shear component, γ+\gamma_+, is about 1.5 (7) times larger than the γ×\gamma_\times uncertainty for targets at i=10∘i=10^\circ (50∘50^\circ). For arbitrary galaxy position angle on the sky, these forecasts apply not to γ+\gamma_+ and γ×\gamma_\times as defined on the sky, but to two eigenvectors in (γ+,γ×,μ)(\gamma_+, \gamma_\times,\mu) space where μ\mu is the magnification. We also forecast the potential of less expensive partial observations of the velocity field such as slit spectroscopy. We conclude by outlining some ways in which real galaxies depart from our idealized model and thus create random or systematic uncertainties not captured here. In particular, our forecast γ×\gamma_\times precision is currently limited only by the data quality rather than scatter in galaxy properties because the relevant type of scatter has yet to be measured.Comment: Accepted to ApJ, 17 pages, 14 figures. Diff from v1: added Sec 3.1 on degeneracies and Appendix with simulations confirming Fisher result

    Shaping Attitudes Toward Science in an Introductory Astronomy Class

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    At many universities, astronomy is a popular way for non-science majors to fulfill a general education requirement. Because general-education astronomy may be the only college-level science course taken by these students, it is the last chance to shape the science attitudes of these future journalists, teachers, politicians, and voters. I report on an attempt to measure and induce changes in science attitudes in my general-education astronomy course. I describe construction of the attitude survey, classroom activities designed to influence attitudes, and give numerical results indicating a significant improvement. In contrast, the literature on attitudes in introductory physics courses generally reports stagnation or decline. I briefly comment on some plausible explanations for this difference.Comment: v2 includes a copy of the surve

    Optical Galaxy Clusters in the Deep Lens Survey

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    We present the first sample of 882 optically selected galaxy clusters in the Deep Lens Survey (DLS), selected with the Bayesian Cluster Finder. We create mock DLS data to assess completeness and purity rates, and find that both are at least 70%70\% within 0.1≤z≤\le z \le 1.2 for clusters with M200≥1.2×1014M⊙M_{200}\ge 1.2\times 10^{14}M_{\odot}. We verified the integrity of the sample by performing several comparisons with other optical, weak lensing, X-ray and spectroscopic surveys which overlap the DLS footprint: the estimated redshifts are consistent with the spectroscopic redshifts of known clusters (for z>0.25z>0.25 where saturation in the DLS is not an issue); our richness estimates in combination with a previously calibrated richness-mass relation yields individual cluster mass estimates consistent with available SHeLS dynamical mass estimates; synthetic mass maps made from the optical mass estimates are correlated (>3σ>3\sigma significance) with the weak lensing mass maps; and the mass function thus derived is consistent with theoretical predictions for the CDM scenario. With the verified sample we investigated correlations between the brightest cluster galaxies (BCG) properties and the host cluster properties within a broader range in redshift (0.25 ≤z≤\le z \le 0.8) and mass (≥2.4×1014M⊙\ge2.4\times 10^{14}M_{\odot}) than in previous work. We find that the slope of the BCG magnitude-redshift relation throughout this redshift range is consistent with that found at lower redshifts. This result supports an extrapolation to higher redshift of passive evolution of the BCG within the hierarchical scenario.Comment: Paper accepted for publication in MNRAS, Table 1 will be available online or under reques

    Brightest Cluster Galaxy Alignments in Merging Clusters

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    The orientations of brightest cluster galaxies (BCGs) and their host clusters tend to be aligned, but the mechanism driving this is not clear. To probe the role of cluster mergers in this process, we quantify alignments of 38 BCGs in 22 clusters undergoing major mergers (up to ∼1\sim 1 Gyr after first pericenter). We find alignments entirely consistent with those of clusters in general. This suggests that alignments are robust against major cluster mergers. If, conversely, major cluster mergers actually help orient the BCG, such a process is acting quickly because the orientation is in place within ∼1\sim 1 Gyr after first pericenter.Comment: accepted to Ap

    Overconfidence in Photometric Redshift Estimation

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    We describe a new test of photometric redshift performance given a spectroscopic redshift sample. This test complements the traditional comparison of redshift {\it differences} by testing whether the probability density functions p(z)p(z) have the correct {\it width}. We test two photometric redshift codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently overconfident (the p(z)p(z) are too narrow) while EAZY produces approximately the correct level of confidence. We show that this is because EAZY models the uncertainty in its spectral energy distribution templates, and that post-hoc smoothing of the BPZ p(z)p(z) provides a reasonable substitute for detailed modeling of template uncertainties. Either remedy still leaves a small surplus of galaxies with spectroscopic redshift very far from the peaks. Thus, better modeling of low-probability tails will be needed for high-precision work such as dark energy constraints with the Large Synoptic Survey Telescope and other large surveys.Comment: accepted to MNRA

    MC2^2: Multi-wavelength and dynamical analysis of the merging galaxy cluster ZwCl 0008.8+5215: An older and less massive Bullet Cluster

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    We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3, Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing analysis, we identify two dominant subclusters and estimate the masses to be M200=5.7−1.8+2.8×1014 M⊙_{200}=\text{5.7}^{+\text{2.8}}_{-\text{1.8}}\times\text{10}^{\text{14}}\,\text{M}_{\odot} and 1.2−0.6+1.4×1014^{+\text{1.4}}_{-\text{0.6}}\times10^{14} M⊙_{\odot}. We estimate the projected separation between the two subclusters to be 924−206+243^{+\text{243}}_{-\text{206}} kpc. We perform a clustering analysis on confirmed cluster member galaxies and estimate the line of sight velocity difference between the two subclusters to be 92±\pm164 km s−1^{-\text{1}}. We further motivate, discuss, and analyze the merger scenario through an analysis of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray surface brightness profile reveals a remnant core reminiscent of the Bullet Cluster. The X-ray luminosity in the 0.5-7.0 keV band is 1.7±\pm0.1×\times1044^{\text{44}} erg s−1^{-\text{1}} and the X-ray temperature is 4.90±\pm0.13 keV. The radio relics are polarized up to 40%\%. We implement a Monte Carlo dynamical analysis and estimate the merger velocity at pericenter to be 1800−300+400^{+\text{400}}_{-\text{300}} km s−1^{-\text{1}}. ZwCl 0008.8+5215 is a low-mass version of the Bullet Cluster and therefore may prove useful in testing alternative models of dark matter. We do not find significant offsets between dark matter and galaxies, as the uncertainties are large with the current lensing data. Furthermore, in the east, the BCG is offset from other luminous cluster galaxies, which poses a puzzle for defining dark matter -- galaxy offsets.Comment: 22 pages, 19 figures, accepted for publication in the Astrophysical Journal on March 13, 201
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